The present invention relates to a method of stir-frying discrete pieces or particles of one or more foodstuffs such as for instance finely cut or diced vegetables or meat as well as an apparatus for carrying out said method and the product produced thereby.
Stir-frying is an important cooking method in the Oriental cuisine for the cooking of finely cut or diced meat and vegetables. In domestic households, stir-frying is usually carried out in a shallow iron pot (Chinese "wok"). In this cooking vessel, the cut raw materials together with cooking oil are subjected to a brief but intense heating procedure under vigorous stirring. The stirring serves not only to mix the ingredients and present new surface portions of the ingredients for being fried by contact with the heated surface of the iron pot but also to scrape the heated surface continuously so that no burning takes place. In the case of vegetables, correct stir-frying yields a tender but firm texture and little exudated water. These two quality criteria are met because of the relatively short cooking time and the concomitant evaporation of water due to the intense heating.
Oriental stir-fried dishes have become increasingly popular in most parts of the world because of their attractive texture and the ease by which stir-fried ingredients can be used as part of ready-made dishes. Examples found in supermarkets today are frozen meat-and-vegetable mixes which can be prepared immediately by stir-frying in the kitchen. In Chinese springrolls the filling mix inside the pancake is also traditionally prepared by stir-frying. Springrolls have become immensely popular as a complete ready-made meal, and the demand has given rise to a large-scale industrial production of springrolls.
The up-scaling of springroll production from household scale to industrial scale is not a trivial problem, provided the producer wishes to match the traditional high quality of springrolls prepared in small scale. One of the problems originates from the difficulties in scaling up of the stir-frying process. Conventional batch cooking vessels cannot yield the necessary high transfer of heat due to an unfavorable low proportion between batch volume and heat transfer area. The result is that the vegetables get a different texture from the desired and rather resemble water-cooked vegetables. In addition, batch cooking is labor intensive and therefore tiring and furthermore requires skill on the part of the cook to achieve consistently high quality end products. There is therefore a need for, on one hand, an industrial-scale continuous stir-frying process and apparatus which can provide the same intense heat transfer as in household cooking so that the desired quality of the stir-fried product can be obtained and, on the other hand, a mechanical batch stir-frying process and apparatus whereby a consistently high quality product may be obtained without substantial manual effort and requiring no special skills while allowing production of relatively large batches of stir-fried product with consistent results.
Most types of continuous cooking equipment are not meant for stir-frying of cuttings but for the heating of starch pastes and other high-viscous foods. For example, a French patent application FR 2,578,153 (1986) describes the cooking of a soft starch paste product in a closed-chamber screw conveyor unit. This process resembles the conventional single screw extrusion cooking process.
Stir-frying is distinguished from deep-fat frying by the fact that stir-fried products are in contact with the heated walls in the equipment, and that only a thin layer of oil covers the heated walls. Furthermore, the stir-frying process is characterized by the food material being particulate, where the particles are continuously overturned so that all faces of the particles can come in brief contact with the heated wall. Without this over-turning of the food pieces they will burn and stick to the walls, and the products will not achieve the uniform, fried, crispy surface that is an essential quality characteristic of stir-fried food.
In a device disclosed in U.S. Pat. No. 3,805,687 the mixing and frying of the ingredients is carried out on an inclined, heated frying surface. A scoop transporter moves to and from along the frying surface scooping up the ingredients from the frying surface and slinging them further along the frying surface towards the discharge end thereof, thereby transporting the ingredients along the frying surface while mixing them and presenting new surface portions thereof for frying contact with the frying surface. This device is in practice only suited for treating relatively large pieces such as French fried potatoes having no tendency to agglomerate, and, moreover, no scraping of the frying surface is performed. Furthermore, the device is relatively complicated and therefore relatively expensive while being difficult to keep clean.
EP-0 779 052-A1 discloses a device having a heating surface in the form of a trough wherein the ingredients to be cooked are transported from an inlet to an outlet thereof by means of a screw conveyor rotatably arranged in the trough fitting closely around the screw conveyor. No provision is made for effectively scraping the heating surface and therefore the heated inner surface of the trough will become caked with material from the ingredients thereby leading to burning of said material and consequent low quality of the end product.